1. Field of the Invention
The present invention relates to silicon-on-insulator (SOI) substrates and semiconductor devices manufactured using SOI substrates. The present invention particularly relates to a bonding SOI technique and also relates to SOI substrates, which are obtained by bonding of a single-crystal or polycrystalline semiconductor layer to a substrate having an insulating surface such as a glass substrate, and semiconductor devices manufactured using SOI substrates.
2. Description of the Related Art
Integrated circuits have been developed, which use a semiconductor substrate called a silicon-on-insulator (SOI) substrate that has a thin single-crystal semiconductor layer over an insulating surface, instead of a silicon wafer that is manufactured by thinly slicing an ingot of a single-crystal semiconductor. When transistors that constitute a part of an integrated circuit are formed using an SOI substrate, parasitic capacitance between drains of the transistors and the substrate can be reduced and a semiconductor integrated circuit can be made to have higher performance. Therefore, SOI substrates have been attracting attention.
As a method for manufacturing SOI substrates, a hydrogen ion implantation separation method is known (for example, see Reference 1: U.S. Pat. No. 6,372,609). The hydrogen ion implantation separation method is a method in which hydrogen ions are implanted into a silicon wafer to form a microbubble layer at a predetermined depth from the surface, the surface into which hydrogen ions are implanted is superposed on another silicon wafer, heat treatment is performed to cause separation using the microbubble layer as a cleavage plane, and a thin silicon layer (SOI layer) is bonded to the other silicon wafer. In addition to the heat treatment for separation of an SOI layer, it is necessary to perform heat treatment in an oxidizing atmosphere to form an oxide film on the SOI layer, remove the oxide film, perform heat treatment at 1000° C. to 1300° C. in a reducing atmosphere to increase bonding strength, and recover a damaged layer on the surface of the SOI layer.
One of the known examples of semiconductor devices using SOI substrates is disclosed by the present applicant (see Reference 2: Japanese Published Patent Application No. 2000-12864). It is disclosed that heat treatment at 1050° C. to 1150° C. is necessary also in that case in order to eliminate trap levels and defects that are caused by stress in an SOI layer.